Propane Conversion With the Wrong Burner Orifices
A technician answers a no-heat call at a rural house that was switched from natural gas to a 500-gallon propane tank about two months ago. The furnace is a 20-year-old, 80% AFUE induced-draft unit rated 90,000 BTU/h input, with a nameplate manifold pressure of 3.5 in. w.c. for natural gas. The propane supplier set the second-stage regulator to deliver 11 in. w.c. at the house. The technician finds the burners still fitted with the factory natural gas orifices, and the conversion label is blank. The burners light with a long, soft, yellow-tipped flame that licks the top of the burner box; the flame lifts and dances when the burner door is opened. Combustion analysis before the flue fully warms shows O2 at 3.1%, CO air-free at 1,150 ppm, and a steady black deposit forming on the burner header and inside the flue collar. The heat exchanger shows no visible cracks. The forecast is 18°F and the homeowner wants heat tonight.
1. Explain why burning propane through natural-gas orifices overfires the burners and degrades combustion. Include the physical reasons behind orifice sizing and primary aeration.
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Orifice sizing is not about pressure alone; it is about how much heat a given volume of gas carries and how that gas entrains primary air. Propane carries roughly 2,500 BTU per cubic foot against about 1,000 for natural gas, and it is also denser (specific gravity near 1.5 versus about 0.6). A natural-gas orifice is drilled large because the gas is lean and light. Feed the same hole with propane at 11 in. w.c. and you push far more heat input through it than the 90,000 BTU/h rating, so the appliance is grossly overfired. At the same time, the burner's primary air openings and the gas velocity that drives air into the burner were sized for a lighter, leaner fuel, so the air-to-fuel ratio collapses. The result is a lazy, luminous yellow flame, heavy CO, and free carbon that plates the header and flue. The absence of a visible crack in the heat exchanger says nothing about soot loading or about what the flue is carrying.
2. The homeowner's handyman suggests simply turning the manifold pressure down to 8 in. w.c. to make the flame smaller. Evaluate that proposal.
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It should be rejected. Lowering manifold pressure does reduce input, but it does not restore the correct orifice-to-fuel relationship, and it makes aeration worse. Dropping pressure reduces the velocity of gas leaving the orifice, which reduces the momentum available to pull primary air into the burner. You end up with the same oversized hole, less gas velocity, less entrained air, and a lazier, more luminous flame, usually with higher CO and more carbon than before. It also puts the appliance outside its listed configuration, because the manufacturer's propane kit specifies an orifice size, a manifold pressure, a regulator spring, and often a marking label as a matched set. Guessing a pressure that produces a pretty flame is not a conversion, and a gas valve is not a derating device. If the correct kit is unavailable tonight, the honest answer is to shut the appliance down and provide temporary heat.
3. List what must be done before this furnace is returned to service, and what you would verify after the conversion.
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Shut off the gas and bar the appliance from service until the work is complete. Install the manufacturer's propane conversion kit: the specified orifices, the gas valve regulator spring if the kit includes one, and the conversion label filled in. Confirm supply pressure at 11 to 14 in. w.c. with the furnace and any other propane appliances running, since an undersized regulator or piping shows up only under load. Set manifold pressure to the LP value on the conversion instructions, typically 10 to 11 in. w.c., and verify it with the burner box closed. Then re-check flame appearance, and run steady-state combustion analysis: O2 in the 4 to 6% range, CO air-free under about 100 ppm and ideally under 50, and temperature rise within the nameplate range. Because the unit sooted, inspect and clean the burners, the heat exchanger, and the flue collar, and confirm the flue is clear. Finally, measure CO in the supply airstream and document every reading and the kit number.